1. Field of the Invention
This invention relates to a method of restoring lost circulation of drilling fluid in a well.
2. State of the Art
Rotary drilling of a borehole is accomplished by rotating a drill string having a drill bit at its lower end. Weight is applied to the drill string while rotating to create a borehole into the earth. The borehole may pass through numerous different strata before the borehole reaches the desired depth. The drill string is usually hollow for reasons which become apparent later. Sections are added to the drill string to increase its length as the borehole is deepened.
This drilling process creates significant amounts of friction which produces heat along with fragments of the strata being penetrated. The fragments of strata must be removed from the borehole and the drill bit must be cooled to extend its usable life. Both of these necessities are accomplished by the circulation of a drilling fluid down through the drill string and up the annulus between the drill string and borehole wall to the surface. Cooling of the drill bit is accomplished by a steady flow of cool fluid from the surface and cutting fragments are carried up the annulus to the surface where they are separated from the drilling fluid.
Drilling fluids typically include water and a complicated mixture of clays, polymeric thickeners and dispersants, and weighting agents. The properties of the drilling fluid are dictated primarily by the physical nature of the geologic formation through which the borehole will be created. The hardness, fluid content, type of fluid, porosity, permeability, in-situ stresses, compressive and tensile strength, and chemical composition are all important characteristics of a geologic formation which impact the formulation of the drilling fluid.
In addition to providing cooling, lubrication and cuttings removal from the borehole, the drilling fluid also provides pressure control for the well. A hydrostatic pressure is produced by the column of drilling fluid in the borehole. The density of the drilling fluid may be varied to produce a hydrostatic pressure at specific points along the borehole which is equivalent to or slightly in excess of the pressure of the exposed geologic formations.
In some cases, the hydrostatic pressure of the drilling fluid column required to control a formation pressure in part of the borehole may exceed the tensile or compressive strength of other exposed formations. If this happens, a fracture may be initiated and propagated in the weaker strata. This causes an alteration in the drilling fluid circulation circuit. Instead of returning to the surface up the annulus, some or all of the drilling fluid will exit the borehole through the created fracture or fractures. This is commonly referred to in drilling practice as "lost circulation".
Lost circulation may be caused by created fractures due to excessive mechanical or hydrostatic pressures applied to exposed strata, natural fractures existing in the strata, or simply by high porosity and permeability within a strata. In all cases, it is important to the drilling process to restore the circulation back to the surface within the borehole. Many materials are used for this purpose and most are bridging additives which can be added to the drilling fluid to block the leak path of the exiting fluid.
Materials such as cottonseed hulls, ground mica, graded sand, walnut or other nut hulls ground to different sizes, diatomaceous earth, gilsonite, ground coal, charcoal, cellophane flakes, and shredded paper are common materials used as lost circulation additives added to drilling fluids. These materials are simply bridging agents and do not form a high strength solid material to seal the leak path.
Portland cement mixtures are often used to provide a more permanent seal than the non-hydraulic drilling fluid additives described earlier. One of the limitations of Portland cement formulations is their susceptibility to drilling fluid contamination. Contamination of the cement slurry with drilling fluid often reduces the compressive strength of the set cement, may greatly extend the setting time of the slurry, and may affect the bonding of the cement to the formation. This is particularly true if the drilling fluid contains significant concentrations of lignosulfonate thinners, carboxylate/hydroxide polymers such as starch, hydroxyethyl cellulose, carboxymethyl cellulose, carboxymethyl hydroxyethyl cellulose, or polyalcohols.
Another limitation of Portland cement mixtures is density. As discussed supra, the cause of the lost circulation problem may be use of a heavy mud which causes fracture initiation or propagation in strata surrounding the borehole. Portland cement mixtures are relatively dense and may simply aggravate this problem. Diluting the Portland cement to reduce its density is not an acceptable solution to this problem since the dilution weakens the Portland cement (i.e. Portland cement is very sensitive to its water/cement ratio).
Yet another limitation of Portland cement is the adverse effect of brine which causes severe strength loss.
Accordingly, the present invention is directed to overcoming the above noted problems in the art, and in particular to problems experienced with effective sealing of strata in oil and gas wells which are causing lost circulation.